Carbon dioxide encourages risky behaviour in clownfish

Carbon dioxide in the ocean acts like alcohol on fish, leaving them less able to judge risks and prone to losing their senses. The intoxication adds to the threats that global warming and ocean acidification pose to marine ecosystems.

Around 2.3 billion tonnes of human-caused CO2 emissions dissolve into the world’s oceans every year, turning the water more acidic.

Philip Munday and colleagues at James Cook University in Townsville, Queensland, Australia, have previously found that if you put reef fish into water with more CO2 than normal in it – similar to the levels expected in oceans by the end of the century – they become bolder and attracted to odours they would normally avoid, including those of predators and unfavourable habitats.

Munday and his colleague Göran Nilsson at the University of Oslo, Norway, have now discovered that CO2 leads to riskier behaviour by interfering with a neurotransmitter receptor called GABA-A.

The pair reared clownfish (Amphiprion percula) larvae in seawater with normal (450 microatmospheres) and elevated (900 microatmospheres) CO2 levels. When they reached adulthood, the fish were given a choice between a water stream containing the odour of common predators such as the rock cod (Cephalopholis cyanostigma) or a stream lacking predatory odours. Those reared in high levels of CO2 swam towards rock cod’s scent around 90 per cent of the time, whereas those that had enjoyed normal levels of CO2 avoided the predator’s scent more than 90 per cent of the time.